lib/vsprintf.c: don't special-case pointers to address null

The %p format of printf() would print a pointer to address null as
"(null)".  This makes sense in a real OS where a NULL pointer must
never be dereferenced, but this is a bootloader, and there are cases
where accessing the data at address null makes perfect sense.

Remove the special case in lib/vsprintf.c using "#if 0" with a comment
to make clear this was an intentional change and to stop re-adding
this code.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>